Speech signal processing is well known in the art and is often utilized to compress an incoming speech signal, either for storage or for transmission. The processing typically involves dividing incoming speech signals into frames and then analyzing each frame to determine its components. The components are then encoded for storing or transmission.
When it is desired to restore the original speech signal, each frame is decoded and synthesis operations, which typically are approximately the inverse of the analysis operations, are performed. The synthesized speech thus produced typically is not all that similar to the original signal. Therefore, post-filtering operations are typically performed to make the signal sound "better".
One type of post-filtering is pitch post-filtering in which pitch information, provided from the encoder, is utilized to filter the synthesized signal. In prior art pitch post-filters, the portion of the synthesized speech signal p.sub.o samples earlier is reviewed, where p.sub.o is the pitch value. The subframe of earlier speech which best matches the present subframe is combined with the present subframe, typically in a ratio of 1:0.25 (e.g. the previous signal is attenuated by three-quarters).
Unfortunately, speech signals do not always have pitch in them. This is the case between words; at the end or beginning of the word, the pitch can change. Since prior art pitch post-filters combine earlier speech with the current subframe and since the earlier speech does not have the same pitch as the current subframe, the output of such pitch post-filters for the beginning of words can be poor. The same is true for the subframe in which the spoken word ends. If most of the subframe is silence or noise (i.e. the word has been finished), the pitch of the previous signal will have no relevance.